In this contribution an analytical approach for estimating the tapping tool’s instantaneous eigenfrequencies of flexural modes is derived. A sensor-integrated tap holder with a close-to-tool vibration sensor attached on the tapping tool is introduced and verified by means of frequency response analysis. The close-to-tool vibration data measured during thread cutting experiments is analyzed in time and frequency domain. The instantaneous eigenfrequencies observed in the spectrogram of the power spectral density are compared with the analytical estimation results. It could be shown that considering for the analytical estimation approach the tapping tool-workpiece contact as clamped boundary condition shows close accordance to the experimental data.